Non-native species invasions

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The introduction of harmful aquatic organisms to new marine environments is believed to be one of the four greatest threats to the world's oceans. An alien or non-native species is one that has been intentionally or accidentally transported and released into an environment outside of its historic or resident geographical range or habitat. Such species are described as 'invasive' if they are ecologically and/or economically harmful. Invasive species can dramatically change the structure and function of marine ecosystems by changing biodiversity and eliminating vital components of the food chain.

Functional integrity of species commuities

Even though all seas are interconnected, there are certain dispersion barriers such as salinity, temperature gradient or ocean currents that keep local communities distinct. Two identical habitats can have different species communities with different types of interactions contributing to the global richness of biodiversity. Indigenous, or native species are those living within their natural range (past or present) including the area that it can reach and occupy using its natural dispersal system. By contrast, introduced species are transported either intentionally or accidentally by human-mediated vectors into habitats outside their native range. These species are also termed as alien, exotic, invasive, foreign, non-native, immigrant, neobiota, naturalized, or non-indigenous. Some biota cannot be sufficiently proved to be neither native nor exotic and these are termed cryptogenic species (Carlton, 1996 [1])

Through the evolutionary fitness processes, species adjust to each other and adapt to available resources by occupying different ecological niches within communities. Virtually all resources are utilized optimally and all available niches are filled, maximising the biodiversity value. Various factors influence the functional integrity of a community. If changes are occurring gradually over a long timescale species have enough time to adapt and fill available niches. In turn more rapid shifts create niche openings and this has been identified as the main prerequisite of species invasion. Non-native species are found primarily in disturbed areas, such as harbours, bays, estuaries and semi-enclosed seas where the communities are weekened by various types of pollution (Elton, 1958[2]; Cohen, 2004[3]).

Vectors of introduction

Whether deliberately or accidentally, people have been transporting whole range of organisms, breaking natural distribution boundaries and interfering with community structures. The unwanted hitchhikers are usually either well hidden or too small to be noticed – for the entire lifespan or just for its part.

For most coastal species the open ocean environment is inhospitable, preventing them from spreading into habitats similar to their own but located elsewhere. Distances separating such habitats might be too long to overcome either through their active swimming abilities or passive floating in water currents. Mechanisms by which humanity aids introduction of exotic species are called vectors of introduction and these are chiefly associated with shipment activities, marine aquaculture or ornamental species trade. Other vectors include the international transport and sale of live marine bait, live seafood, and live organisms for research and education.

  • Shipment

Organisms can travel either attached to the submerged part of the vessels hull or contained within their ballast waters. Ballast water has been extensively used since 1870s and certain species are able to complete their life cycle and breed in ballast waters, meaning that they can be translocated far away from their native range. Studies on ballast tanks found more than 1,000 taxa ranging from phytoplankton to small fish up to 15 cm in length (Gollash, 2004[4]).

  • Mariculture

There has been a growing interest in the search for fish, shellfish, and plant species whose biology was well known and which either already had achieved or could achieve success in cultivation. Depending on the type of mariculture, the organisms can be either allowed to establish in the wilderness (intentional species introduction) or kept in enclosures from which they occasionally escape (accidental release) either directly or through their dispersal system. Very often those new introduced species carried along parasitic organisms that could later establish themselves in indigenous species. Since the latter have not developed any defence strategy against non-indigenous parasites, they could be greatly impacted.

  • Ornamental species trade

Saltwater species are a rapidly growing sector of the aquarium industry. Species can be released by their owners either accidentally or deliberately when no longer wanted. This vector is responsible for one of the major invasions launched up to date, namely the release of Caulerpa taxifolia from Oceanographic Museum of Monaco.

  • Indirect introduction

Occasionally, species introduction can result not from their physical re-location but by offering a way for its dispersal to areas that it wouldn’t be able to reach if the conditions were not changed. Such opportunity is offered by alteration of hydrological regimes, like canal and reservoir construction (Grigorovich et al., 2002[5]).

  • Secondary introduction

If species are introduced and become established in a new geographic setting, they might continue to spread by both natural and anthropogenic mechanisms, colonizing habitats that they wouldn’t be able to reach without the initial, human-mediated translocation. Such introduction is termed a secondary introduction.

Very rarely we can connect an invasion event with one particular vector. Instead it is more common to assess the probability of a given vector being responsible. In British waters accidental release associated with mariculture has been identified as the main vector with other important vectors being fouling, ballast water and deliberate commercial introduction (Eno et al., 1997[6]).

From establishment to invasion

Not all translocated species become established in new environments. If the population is relatively small it will be vulnerable to stochastic threats such as demographic or genetic drift. It is hard to predict what is the minimum viable population size. A generally accepted rule is that 50 individuals are needed to prevent excessive inbreeding and a minimum population of 500 hundred is required to keep a sufficiently high level of heterozigosity (Franklin 1980[7]). Yet, those numbers may very greatly between different taxa and even if the population is large enough to prevent loss of alleles, it still might be prone to environmental threats such as poor food or oxygen availability, impacting recruitment success or juveniles development, or catastrophic events (El Niño, tsunami, etc.)

If the founder population is large enough to overcome stochastic threats and manages to establish itself in a new environment, it will join the network of interactions within the receiving environment. Alien species have evolved in different environments and it is hard to predict what their interactions with indigenous biota will be. Not all introduced species can be termed as invasive. Some of them may well coexist with native species and share the resources. But every now and then an introduced species becomes an invader and impacts the host community, with the ultimate result being destabilization of the system and possibly extinction of native species.

Organisms have been carried around hidden in dry ballast, attached to hulls or buried inside them for millennia and it is very likely that what we now consider cosmopolitan species are simply early introductions and that species such as ship-boring isopod (Sphaeroma terebrans), Asian seasquirt (Stylea plicata), giant kelp (Macrocystis pyrifera), mussels (Mytilus galloprovincialis and Mytilus edulis) and European periwinkle (Littorina litorea) can be possible early introductions (Carlton, 1999[8]). This in turn raises the issue of naturalisation: How much time is needed to fully incorporate a population in a community and bring the latter to equilibrium? How many interactions have to be established to call an introduced species integrated? Carlton also questioned the quality of our understanding of present marine communities. If we assumed a rather timid scenario that between 1,500 and 1,800 only three species were introduced elsewhere each year, we would have ended up with a number of 1,000 species that might have spread before humanity gained a general knowledge of biogeography, taxonomy and ecology. As a result, these species can be today considered as cosmopolitan. It might have a great impact on our understanding of marine ecology and community equilibrium in particular.


Introduced colonial sea squirt (Didemnum sp.) fouling other organisms, including the commercially important blue mussel Mytilus edulis in Irish Sea. (Photograph taken by Mr. Tom Ohman)
  • Competition

Introduced species might have evolved in the presence of much more aggressive competitors than the ones present in the receiving environment and they might be extremely successful in colonising new areas. The most common competition is for resources, whether it is food, solar energy or space. In California bay the native shore crab (Hemigrapsus oregonensis) declined in mean abundance by 10 times as a consequence of the European green crab (Carcinus maenas) introduction and competition for food (Nutricola sp.) (Grosholz et al., 2000[9]).

  • Diffuse competition

The impact on native organisms can be greater through diffuse competition. A population might resist competition along one axis if the resource is plentiful, but its realised niche can virtually disappear in the presence of several species competing along many axes (Giller, 1984[10]).

  • Positive indirect interactions

Even with a stable or decreasing rate of new introductions, it can be assumed that systems will become increasingly destabilized through positive indirect interactions and diffuse competition, as more and more interactions within the community become altered. Even early, non-invasive introductions may become a nuisance due to a change induced by another introduced species (Grosholz, 2005[11]). It is possible for a new alien to transform an older exotic species into an invader.

  • Predation and herbivory

Some organisms are more effective competitors for resources than others and have the potential to dominate the area, excluding other species from the access to viable resources. In such cases organisms feeding on it (predators or herbivores) act as a biocontrol agent keeping their abundance low and maintain a high level of stability and productivity. If those predators or herbivores are introduced to a new environment, local community members might be lacking the evolutionary-driven ability to resist them. On the other hand if a species limited in number in its native range by biocontrol agents is introduced to a new environment, it might thrive and outcompete indigenous species.

  • Parasitism

Very often introduced organisms are accompanied by parasitic species. Mass transfer of large numbers of animals and plants without inspection, quarantine, or other management procedures has led to the simultaneous introduction of pathogenic or parasitic agents.

  • Genetic impact

If transferred species are closely related to native counterparts, they may hybridise affecting the original gene pool. Introduced or hybridised individuals may prove to be more successful in mating than the indigenous organisms and the original genetic diversity might be lost.

  • Multiple negative interactions

Two species may interact with one another in a several ways. A species confronted by one-way interactions can compromise and trade-off one component of fitness for the sake of the survival. If it is forced to compromise too many components it may be pushed into an extinction vortex (Russel et al., 2004[12]).

The problem

The comb jelly Mnemiopsis leidyi (native to western Atlantic) feeds on eggs and larvae of pelagic fish and caused a dramatic drop in fish populations in the Black Sea by competing for the same food sources and eating the young and eggs (Kube et al., 2007[13])[14]

Consequences of human-induced altering of species composition are sometimes detrimental to local communities and their magnitude may range from limitation or exclusion of single species or to destabilization of the whole system and species introduction has been identified as one of the main causes of species extinction. Even with a stable or decreasing rate of new introductions, it can be assumed that systems will become increasingly destabilized through direct and indirect interactions and diffuse competition as more and more interactions within the community become altered. In areas that have already been heavily invaded, reducing the numbers of new introductions may not be a sufficient strategy. In addition to preventing new introductions, it may be necessary to mitigate the impacts of exotic species that have already become established. Given the large number of alien species already present, there is a high potential for positive interactions to produce many future management problems.

At least 50 non-native species have entered the Black Sea in the last century and some have been invasive. For instance, the comb jelly-fish Mnemiopsis leidyi was the primary cause of collapse of the fisheries in the area in the early 1990s. Over 100 non-native species have been recorded in the NE Atlantic, mainly in the North Sea, the Celtic Sea, the Bay of Biscay and along the Iberian coast. Impacts of invasive species vary in different regions and sometimes are rather localized. Over the past twenty years, the number of alien species transported into the Baltic has increased and poses a significant threat to the region given its naturally low species diversity. In North America, some 300 non-indigenous species of invertebrates and algae have been established in marine and estuarine waters (Ruiz et al., 2000[15]). Rates of invasion among most microscopic organisms such as bacteria are still unreported and it appears that their potential of dispersal by human-mediated vector is very significant.

Further reading


  1. Carlton, J.T. (1996). Biological invasions and cryptogenic species. Ecology 77(6): 1653-1655.
  2. Elton, C. S. (1958) The Ecology of Invasions by Animals and Plants, Methuen, London.
  3. Cohen, A. N. (2004). Invasion in the sea. In: Park Science vol. 22, no. 2, fall 2004 pp. 37-41
  4. Gollash, S. (2004). A global perspective on shipping as a vector for new species introduction. Presented at the 13th International Conference on Aquatic Invasive Species, Ennis, Co Clare, Ireland 2004. Institute of Technology, Sligo.
  5. Grigorovich, I. A., MacIsaac H. J,. Shadrin, N. V., Mills, E. L., (2002). Patterns and mechanisms of aquatic invertebrate introductions in the Ponto-Caspian region. Canadian Journal of Fisheries and Aquatic Science 59: 1189-1208.
  6. Eno, N. C., Clark, R. A., Sanderson, W. G. (eds), (1997) Non-native marine species in British waters. Joint Nature Conservation Commission, Peterborough, UK.
  7. Franklin, I. R. (1980) Evolutionary change in small populations. In M. E. Soulé and B. A. Wilcox (eds) Conservation Biology: An Evolutionary-Ecological Perspective, pp. 135-39. Sinauer Associates Sunderland, Massachusetts.
  8. Carlton, J. T. (1999). Quo vadimus exotica oceanica? Marine bioinvasion ecology in the twenty-first century. In Pederson,. J. (ed.), Marine Bioinvasions: Proceedings of the First National Conference, January 24-27, 1999. Massachusetts Institute of Technology, Cambridge.
  9. Grosholz, E. D., Ruiz, G. M., Dean, C. A., Shirley, K. A., Maron, J. L. & Connors, P. G. (2000). The Impacts of a Non-indigenous Marine Predator in a California Bay, Ecology 81, 1206–1224.
  10. Giller, P. S. (1984) Community Structure and the Niche. Chapman and Hall, London.
  11. Grosholz, E. D. (2005) Recent biological invasion may hasten invasional meltdown by accelerating historical introductions. Proceedings of the National Academy of Sciences of the United States of America, 102 (4): 1088-1091.
  12. Russel, B.R., Mills M. D., Belk, M. C. (2004). Complex interactions between native and invasive fish: the simultaneous effects of multiple negative interactions. Presented at the 13th International Conference on Aquatic Invasive Species, Ennis, Co Clare, Ireland 2004. Institute of Technology, Sligo.
  13. http://Kube, Sandra; Postel, Lutz; Honnef, Christopher & Augustin, Christina B. (2007): Mnemiopsis leidyi in the Baltic Sea - distribution and overwintering between autumn 2006 and spring 2007. Aquatic Invasions 2(2): 137-145.
  15. Ruiz, G.M., Fofonoff, P.W., Carlton, J.T., Wonham, M.J. and Hines, A.H. (2000). Invasions of coastal marine communities in North America: apparent patterns, processes, and biases. Annual Review of Ecology and Systematics, 31: 481-531.

The main author of this article is Penk, Marcin
Please note that others may also have edited the contents of this article.

Citation: Penk, Marcin (2019): Non-native species invasions. Available from [accessed on 15-10-2019]

Introduction to Restoration and preservation of coastal biodiversity

This article summarizes knowledge and practices in Europe on the valuation of coastal and marine biodiversity. Through the concept of Biological Valuation and Biological Valuation Maps (BVM) that can be used as baseline maps for future spatial planning at sea, practical guidelines for incorporating biodiversity in coastal and marine policies are provided. See articles in the category Marine_Biodiversity.


Biodiversity is an all-inclusive term to describe the total variation among living organisms of our planet. In its most simple form, biodiversity or biological diversity is therefore 'Life on Earth' and marine biodiversity is 'Life in the Seas and Oceans. While genetic, species and ecosystem diversity can be considered as elements of ‘structural diversity’, ‘functional diversity’ refers to the variety of biological processes, functions or characteristics of a particular ecosystem. There are several ways in which ecological classifications group organisms according to common functions: classification according to their habitat, to their position in the food web or to their functional feeding mechanism. The Coastal Wiki provides the most commonly used measurements of biodiversity (species richness, evenness and taxonomic indices) and sampling tools (both for pelagic and benthic organisms) of marine biodiversity.


National and international strategies for the biological valuation

The continuously increasing socio-economic pressure on the coastal system urges the need for a decision-making framework to objectively allocate the different user functions in the coastal zone. The coastal biodiversity suffers badly from the anthropogenic stress hampering the urgent need for a thorough preservation and restoration of coastal biodiversity. A strategy is needed to provide an integrated view on nature’s intrinsic value. Biological value is here defined as the value of biodiversity, without any reference to anthropogenic use. As such, the biological value complements the social and economic valuation. Till now, when requested, the biological value of an area was basically assessed through an unguided procedure, primarily based upon a (the available) best expert judgement. A marine biological valuation strategy, in contrary, should ideally be (1) scientifically widely acceptable, to avoid an uncontrolled proliferation of valuation strategies (i.e. broad scientific support), and (2) widely applicable, to maximise its applicability (e.g. stakeholder involvement). In order to support such a strategy, Biological Valuation Maps (BVM) that compile and summarize all available biological and ecological information for a study area, and that allocate an overall biological value to subzones can be used as baseline maps for future spatial planning at sea.

Concept of biological valuation

There is worldwide recognition of the benefits of management for sustainable use and conservation of the sea. In order to develop management strategies in the marine environment, reliable and meaningful, but integrated ecological information is needed. Marine biological valuation maps that compile and summarize all available biological and ecological information for a study area, and that allocate an overall marine biological value to subzones, can be used as baseline maps for future spatial planning at sea. Biological valuation assessments have been developed primarily for terrestrial systems and species. Problems encountered when applying terrestrial-based assessments to marine areas are currently demonstrated in the difficulties encountered implementing the EC Habitats Directive (92/43/EEC) in the marine environment. The Directive was written from a terrestrial viewpoint, and applying it to more dynamic marine systems has proved problematic. Criteria developed for identifying terrestrial species and habitats for conservation cannot be easily applied to the marine environment. Therefore, different valuation criteria are needed for marine areas. Therefore, the European Commission has developed a Marine Strategy Directive, which recognizes the need for a thematic strategy for the protection and conservation of the European marine environment with the overall aim to promote sustainable use of the seas and conserve marine ecosystems. It is necessary that the definition of the value of marine areas should be based on the assessment of areas against a set of objectively chosen ecological criteria, making best use of scientific monitoring and survey data. A first step towards such an objective valuation framework was made in the Netherlands, where selection criteria from the EC Habitat and Bird Directives and the OSPAR guidelines were used to determine which marine areas have special ecological values in terms of high biodiversity. A scientifically sound and widely applicable concept for marine biological valuation, drawing on existing valuation criteria and methods [1] and attempts to rationalize them into a single model was performed. This concept represents a consensus reached by a large and diverse group of experts in the field.

The valuation criteria that were selected for this concept are rarity, fitness consequences, aggregation (as first-order criteria), naturalness and proportional importance (as modifying criteria). Once the concept of biological valuation is applied to a marine study area, the result of this process could be visualized on marine biological valuation maps (BVMs). Marine BVMs can act as a kind of baseline describing the intrinsic biological and ecological value of subzones within a study area. They can be considered as warning systems for marine managers who are planning new, threatening activities at sea, and can help to indicate conflicts between human uses and a subzone’s high biological value during spatial planning.


Adaptation of the Concept[2]

During a workshop the concept and protocol of marine biological valuation was discussed, which resulted in fine-tuning the concept of marine biological valuation, especially with respect to its applicability to marine areas. The concept of marine biological valuation was reorganized to avoid double counting of scores (i.e. lumped criterion ‘aggregation-fitness consequences’) and to allow a more logical order of the steps which should be made during valuation (i.e. assessing the biological value on two different scales instead of incorporation of ‘proportional importance’ as a valuation criterion). ‘Rarity’ was retained as a valuation criterion while ‘naturalness’ was excluded from the concept since ‘naturalness’ is usually assessed on the basis of the absence of human impacts in the subzone. This makes it almost impossible to apply this criterion without specific reference to human impacts, which is deliberately excluded from the definition of biological valuation.

Wiki-link: Workshop Marine biological valuation]

Development Protocol (tools)[3]

Guidelines for a generic biological valuation protocol based on the above mentioned valuation criteria has been set up and tested on a European scale. The steps in the valuation protocol are described are given below. They encompass the selection of the valuation criteria for the determination of the appropriate assessment questions, the practical algorithms to evaluate the criteria and the final scoring of all assessment questions.

  • Select valuation criteria at two different scales, first at the local (study area) scale and secondly at a broader, (eco)regional scale.
  • Subdividing the study area in subzones. For the purpose of marine biological valuation a division of the study area in subzones according to a habitat classification seems most appropriate.
  • Available data and reliability of information. A detailed database, covering all data and information used for the value assessment, should be attached to the maps, and this should be consulted whenever the maps are used to guide advice or when used as a warning system in management decisions.
  • Assessment questions. By answering a set of possible assessment questions, related to the different structures and processes of biodiversity and coupled to the proposed valuation criteria, all aspects linked to biological and ecological valuation are visualized
  • Mathematical algorithms. By developing specific algorithms for each assessment question the value of the subzones can be quantitatively assessed relatively to each other.
  • Scoring. It seems impossible to set uniform thresholds which would be applicable to all marine ecosystems, so this needs to be done on a case by case basis. When all relevant questions are scored for the different subzones within a study area, all criteria (with respect to all organizational levels of biodiversity) are assessed.
  • The results of the biological valuation of a study area can now be presented on a map, where each subzone within the area is assigned a colour corresponding with its value [4].

A major benefit of the proposed marine biological valuation protocol is the fact that all available biological and ecological data are integrated for each subzone, which makes the comparison between subzones easier for the users of the Biological Valuation Maps. The reliability of the assessed intrinsic value should be noted by attaching a label to the different subzones. This label can display the amount and quality of the data used to assess the value of a certain subzone or it can display how many assessment questions could be answered given the data available for each subzone (reliability of information). These reliability labels should be consulted simultaneously while using the BVMs. Next to that, they help to identify knowledge gaps which could direct future scientific research.

Threats to biodiversity

Four main treats to marine biodiversity: Eutrophication, Fisheries, Invasions and the Effects of Global Climate Change.


In the EEA report State of Europe's seas [5] an overview is given of the most important physical, biological and exploitation characteristics, the main threats to biodiversity and the policies at work (nature protection and protection of marine resources by restrictions on fishing and hunting).


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  2. Derous S, Austen M, Claus S, Daan N, Dauvin J-C, Deneudt K, Depestele J, Desroy N, Heessen H, Hostens K, Marboe AH, Lescrauwaet A-K, Moreno M, Moulaert I, Paelinckx D, Rabaut M, Rees H, Ressureiçao A, Roff J, Santos PT, Speybroeck J, Stienen EWM, Tatarek A, Ter Hofstede R, Vincx M, Zarzycki T, Degraer S (2007). Building on the concept for marine biological valuation with respect to translating it to a practical protocol: Viewpoints derived from a joint ENCORA-MARBEF initiative. Oceanologia 49(4): 1-8
  3. Derous S, Courtens W, Deckers P, Deneudt K, Hostens K, Moulaert I, Paelinckx D, Rabaut M, Roff JC, Stienen EWM, Van Lancker V, Verfaillie E, Vincx M, Degraer S (submitted). Biological valuation: Guidelines for a transparent and generally applicable protocol for the marine environment Aquatic Conservation: Marine and Freshwater Ecosystems
  4. Derous, S. 2008. Marine biological valuation as a decision support tool for marine management. Thesis Gent University
  5. EEA 2015. State of Europe's seas.Report No 2/2015, ISBN 978-92-9213-859-2, doi:10.2800/0466, 216 pp